Band Gap Engineering of Titanium dioxide nanotubes for solar Energy conversion – BAGETE

The project aims to improve by different methods the properties of titanium dioxide (TiO2) which is known for its good stability for PEC applications but whose performances are limited by its wide band gap allowing only the absorption of UV radiation (< 5% of the solar spectrum). To increase its performances, this work will explore new approaches to modify the morphology and electronic properties of TiO2. First, by making 1D nanostructures, to benefit from their good photo-generated charge transport properties. A large part of the project was dedicated to improving these TiO2 nanostructures using an original «co-alloying« method. This approach aims to introduce a large proportion of anions and cations in substitution of O2- and Ti4+ in the crystal lattice of TiO2. By choosing appropriate anion/cation pairs, it is possible to modify the electronic structure of TiO2 in order to reduce its band gap for a better light absorption. In addition, the charge balance leads to a more stable crystal structure and with fewer defects than with conventional doping methods. This last point is crucial in order to have good mobility of the charge carriers and to limit their recombination.
The other modification of TiO2 nanotubes studied in this project is the deposition of catalyst nanoparticles on the surface of the nanotubes. For this, we will use and compare different physicochemical methods to deposit oxides of non-noble metals (Co, Ni) known for their catalytic properties while being less expensive than the Platinum group metals generally used for this purpose.

The main results that could be drawn from the project were the validation of the co-alloy strategy: we were able to show that the insertion of Nb5+ and N3- as doping species makes it possible to absorb a part of the visible light contributing for approximately 20% of the overall photo-electrochemical conversion of the sample. For the co-catalyst deposition part, we have shown that Cobalt and Nickel both increase the efficiency of the reaction and identify an additional advantage of Co which is directly participating in the photo-electrochemical reaction in the visible. Finally, the combination of the two approaches is very interesting and allows the production of hydrogen to be tripled.

The BAGETE project aims to improve photo-electrodes based on TiO2-NTs using co-alloying approach and co-catalyst deposition. Both approaches bring improvement to the system, co-alloying thank to an improvement of the visible light conversion, co-catalyst deposition using Ni or Co oxide nanoparticles improved the overall efficiency of the reaction. Interestingly, combining the 2 approaches increases furthermore the efficiency by a synergic effect still to be understood. This allows to triple the photocurrent but also the hydrogen production as measured online by µGC.

During the project new aspects arise and should now be taken into account for future development: combining different approaches to improve the PEC efficiency as proven interesting but it results in complex heterostructures that need to be optimized. In view of the numerous properties that can be affected by each component of this heterostructures, it is important to develop in operando characterization methods to determine the limiting steps of the reaction, but also methods to optimize rapidly complex photo-electrode materials.

This work has been exposed in 4 articles published in international peer-reviewed scientific journals as well as by presentations at various national and international conferences and congresses. The results obtained have opened up new perspectives which should lead to new collaborative projects in the process of being set up.

Publications :
1. Comparative study of the photocatalytic effects of pulsed laser deposited CoO and NiO nanoparticles onto TiO2 nanotubes for the photoelectrochemical water splitting.
T. Favet, T. Cottineau, V. Keller, M. A. El Khakani.
Solar Energy Mater. & Solar Cells, 217, 2020, 110703

2. Electrosynthesis of gradient TiO2 nanotubes and rapid screening using scanning photoelectrochemical microscopy
F. Gelb, Y.-C. Chueh, N. Sojic, V. Keller, D. Zigah, T. Cottineau.
Sustainable Energy & Fuels, 4, 2020, 1099-1104.

3. Enhanced visible-light-photoconversion efficiency of TiO2 nanotubes decorated by pulsed laser deposited CoNi nanoparticles
T. Favet, V. Keller, T. Cottineau, M. A. El Khakani.
International J. of Hydrogen Energy, 44, 2019, 28656.

4. Influence of the anatase/rutile ratio on the charge transport properties of TiO2-NTs arrays studied by dual wavelength opto-electrochemical impedance spectroscopy
T. Cottineau, H. Cachet, V. Keller, E. M. M. Sutter.
Phys.Chem.Chem.Phys. 19, 2017, 31469-31478.

The BAGETE project will focus on the development of nanostructurated metal oxide electrodes for their use as photo-anodes for hydrogen production by Photo-ElectroChemical (PEC) water splitting. This project fits in the frame of the Challenge 2 “Clean, safe and effective energy” for the Young Scientist competition.

Solar energy is an attractive renewable energy source with low environmental impact. However, it remains a challenge to produce a continuous flow of usable energy from sunlight, due to the diffuse and fluctuating nature of the solar irradiation. Therefore this project aims at developing new materials that can directly convert solar energy into energetic chemical species, also called “solar fuels”, which can be stored and distributed on demand. The PEC approach combines, in a single structure, a light absorbing material (usually a semiconductor), and a catalytic part for the redox reactions. The principle of PEC cell has been confirmed for production of H2 and O2 by water splitting at the laboratory scale. However among all the different materials and architectures tested, none is totally satisfying yet. This is due to the fact that it is difficult to combine high solar-to-chemical energy conversion efficiency with stability of the semiconducting material toward photo-corrosion.

The project will use titanium dioxide as a starting material. TiO2 is known for its good stability for photo-electrochemical applications. However its performances are largely limited by its wide bandgap that only allows absorption of UV light. Therefore in order to improve its performances, this work will focus on developing novel synthesis methods that can modify the morphology and the electronic structure of TiO2. The first objective will be to synthesize 1D aligned TiO2 nanostructures which will improve the photo-generated charge transport. Then, a large part of the project will be devoted to the development of original co-alloyed TiO2 structures. This approach aims to introduce large atomic percentage of cationic and anionic species in substitution of Ti4+ and O2- in the TiO2 lattice. This insertion should be stoichiometric to achieve a balance of the charge between anions and cations. By an appropriate selection of anions/cations pairs, we can modify the electronic band structure of TiO2, with the aim to reduce its bandgap for a better solar light absorption. Furthermore, thanks to the charge balance, the co-alloying approach will provide a more stable crystalline structure with fewer defects than the classical mono-doping approaches. This last point is important to preserve a high mobility of charge carriers and avoid their recombination.

Cationic species insertion will be achieved simultaneously with the TiO2 nanotubes growth by an original in-situ method while the anion insertion will be achieved by adapted thermal treatments. Specific characterization methods will be developed to explore the properties of the co-alloyed materials, especially their crystalline structure (TEM with cartography, XRD) and electronic properties (photoluminescence, impedance spectroscopy). Ultimately, the knowledge we will gain on co-alloying method will be used to synthesize TiO2-NTs photo-electrodes with variable co-alloying elements concentrations, in order to absorb photon with different energies gradually in the thickness of the film. We expect these original structures will provide a better light absorption with efficient transport of charge carriers.

To further improve the PEC properties of the co-alloyed TiO2 photo-electrodes, we will deposit catalytic nanoparticles on their surface to enhance the charge transfer and the overall efficiency of the reaction. Finally, the modified TiO2 nanostructures will be tested in PEC experiments in different conditions (such as irradiation, electrolyte pH) to identify the best approaches and modifications to reach stable and highly efficient solar-to-chemical energy conversion.